AJP - Endocrinology and Metabolism, Vol 267, Issue 2 E210-E218, Copyright © 1994 by American Physiological Society
Muscle glyconeogenesis during recovery from a prolonged swim in rats
C. Ryan and J. Radziuk
Diabetes and Metabolism Research Unit, Ottawa Civic Hospital, University of Ottawa, Ontario, Canada.
Glyconeogenesis in muscle was assessed during a 3-h recovery period after
prolonged submaximal exercise represented by a 4-h swim. Rats fasted for 12
h and previously catheterized underwent this protocol with the concomitant
infusion of [6-3H]glucose and one of the following: 1) [14C]bicarbonate, 2)
[U-14C]lactate, and 3) [1-14C]glucose. Rested rats served as controls. The
incorporation of 14C label ([14C]bicarbonate and [U-14C]lactate) or its
transfer to the sixth position of glucosyl units of glycogen, over and
above that taken up from circulating glucose (and determined from
[6-3H]glucose uptake), was used as an index of muscle glyconeogenesis. 14C
from 14CO2 is not expected to be incorporated into glycogen in muscle, and
any incorporation that is not from circulating glucose is used to define
experimental error. [14C]lactate incorporation measures equilibration with
circulating lactate, and label randomization in glucosyl units beyond that
seen in plasma glucose is taken as evidence of glyconeogenesis from locally
accumulated glycolytic products. The results of these studies demonstrate
1) no glyconeogenesis in the soleus; 2) in the red and white gastrocnemii,
glyconeogenesis takes place only from glycolytic products within the
muscle. Approximately 35-40% of the [6-14C]glucose in glycogen can only be
accounted for by muscle glyconeogenesis. The substrate does not equilibrate
with circulating lactate to a detectable extent. 3) Glyconeogenesis appears
to persist throughout the recovery period and uses substrate at the level
of pyruvate. This is consistent with a continuing elevation of glycolysis
during this period.
